Latina girls have higher teen birth rates than girls from any other ethnic group and higher rates of sexually transmitted diseases (STD) than non-Latino Whites. Given their high rates of teen pregnancy and STDs, studying the sexuality of Mexican American adolescent girls is crucial. Research has shown that siblings play an influential role in youth adjustment and that older sisters, in particular, are important socializing agents of girls'sexuality, yet few studies have focused on the specific mechanisms and processes by which older siblings influence their younger sisters'sexual experiences and attitudes. This study seeks to fill that void by addressing two specific aims. The first aim of the proposed research is to investigate concurrent and longitudinal similarities and differences in Mexican American sisters'sexual experiences and attitudes and to examine the mechanisms (specifically, sibling modeling and de-identification) by which older and younger siblings'sexual experiences and attitudes are alike or different. The second aim of the proposed research is to examine sexual communication between siblings as a mechanism linking older and younger sisters'sexual experiences and attitudes. A sample of 75 Mexican American sister-sister dyads will be recruited. Based on social learning and sibling de-identification frameworks, both survey and observational (i.e., videotaped conversations) data collection methods will be employed to investigate the specific mechanisms whereby older sisters may act as socializing agents of their younger sisters'sexual experiences and attitudes. Information from the proposed study will inform culturally appropriate sexual health programs that will be targeted to adolescent girls of Mexican-origin. Further, the findings from this study also will reveal the role of older sisters in teen pregnancy and STD prevention programs.